In large scale manufacturing and assembly plants, such as those used in automobile manufacturing, hundreds of machines and their machine operators may work simultaneously. In a large production environment, the production line may include miles of conveyors. The plant itself may be millions of square feet. An increase in the precision of production timing and/or control may provide better resource allocation. Accordingly, process and controls that keep the line moving may increase production and reduce expenses.
A plant's hours of operation may include three shifts. For example, at the beginning of the day, oftentimes there is work in progress left at the end of the previous shift. Work in progress includes but is not limited to units of production, pallets, carriers, product, parts and units of production and items of production. In a typical plant-floor manufacturing environment, it is the line supervisor's responsibility to promote modifications to the system to optimize product throughput. Throughput is the number of items produced or processed during a period of time. Prior to beginning each shift the line supervisor may manually redistribute the work in progress so that operations throughout the shift will run more smoothly than if production started with work in progress where it was at the end of the previous shift. However, in large systems (where the supervisor cannot see the entire system) it becomes almost impossible for the supervisor to see and/or control product or unit of production location(s) to maximize throughput. Therefore, at the beginning of shift, oftentimes the shift will indeed start with the parts where they were left by the shift that just ended.
Along the many sections of conveyor of a large plant, there may be a plurality of sections that are empty or full. A buffer is storage area such as a conveyor, transport (i.e. forklift, cart, truck, etc.), silo between stations (either manual or automatic), or simply a place on the floor. A buffer generally can hold parts in excess of standard in-process stock, and provides capability to cover potential production losses due to downtime, lost time, and quality issues. Here, standard in-process stock means the minimum number of parts on the line required to maintain continuous flow of products through the manufacturing line at the intended line speed.
If a buffer is empty, the station drawing from that buffer is starved. If a buffer is full, the station feeding that buffer will be blocked. Because of the difficulty in manually managing the contents of the buffers, line supervisors traditionally allow buffers (which may include pallets holding large units of production such as engines or entire automobiles in different stages of assembly), to become congested during the manufacturing day. This congestion, or fullness, causes starvation in some areas of the line while causing blockage or congestion in other areas.
As the line balance deteriorates throughout the day supervisors will often manually manage the system workflow using time (i.e., breaks and lunches), resources (i.e., working certain sections of the line while furloughing other sections), or functionality of the line (i.e., potential buffering to support loading and unloading of product from the line). This manual approach to workflow management leads to a system that changes based on the then present circumstances. Production is therefore less predictable than desired, and costs for manpower may not be well managed.
In a single shift, a machine station at an automotive plant may process up to hundreds even thousands of products. The conveyor belt may move at several feet per second. The line moves fast and operations are complex. In a single automotive assembly line, there may be hundreds of machine stations and zero or more machine operators per station. Stopping a unit of production for staging or choosing to let the unit of production move on a conveyor belt under the continuous flow model is a decision that can cause problems if the wrong approach is taken. Since stopping a unit of production from moving takes typically 0.5 seconds over takt time, units of production can become bottlenecked quickly and downstream machines can be starved. Over the course of processing 1000 units of production per shift an ad hoc and manual approach may mean substantial losses in time and resources.